The problem with enums is that neither is_integral, nor is_signed or
is_unsigned is true for them. Thus, comparison with our CHECK/DCHECK
macros always just uses the default comparison, which fails if the
signedness of the underlying integer types does not match.
This CL fixes this by considering the underlying integer type of an
enum to choose the right comparison operator.

R=ishell@chromium.org
CC=ahaas@chromium.org

Change-Id: I5ef56d1b86228e879f5866967ab7e709f1e97f0b
Reviewed-on: https://chromium-review.googlesource.com/518123
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45593}